An apparatus of the aforementioned type is known, for example, from U.S. Pat. No. 3,738,341. In this application, a sensor continuously analyzes the exhaust gases of an internal combustion engine with respect to the air-fuel ratio, with this ratio being corrected in accordance with the result of the analysis by suitably regulating the supply of fuel or air. The sensor for analyzing the exhaust gas is responsive to the oxygen content of the exhaust gas and has its most effective operating range at temperatures of between 400.degree. and 500.degree. C.
Sensors of this type, which respond to the oxygen content of the exhaust gas (Lambda sensors) including the pattern of the sensor output voltage in dependence on Lambda, are disclosed, for example, in U.S. Pat. No. 4,345,562. The sensor includes a solid electrolyte, for example, zirconium dioxide, to which contacts are applied to both sides. If the oxygen proportions at the two surfaces of the solid electrolyte differ, a potential difference results at the contacts which changes abruptly at an air ratio of Lambda=1. This voltage change of the Lambda sensor occurring at Lambda values .lambda.=1 is conventionally utilized for control purposes because the voltage change is relatively independent of other parameters such as temperature and can be reliably detected using threshold-value switches.
Further, methods and apparatus are known that serve to compensate for offset-voltage influences in operational amplifiers. The book "Circuits for Electronics Engineers", S. Weber, McGraw-Hill, Inc., New York 1977 describes on page 243 an arrangement including two operational amplifiers connected in series, in which the influences of the offset voltage and offset-voltage drifts compensate each other. However, this method promises to be successful only if the spread between the two operational amplifiers is neglected.
The above-described state-of-the-art control arrangement including an oxygen sensor to detect the air-fuel ratio operates satisfactorily as long as the change in the sensor output voltage at .lambda.=1 is utilized for the control.
In various cases, it may however be advantageous with a view to achieving optimum exhaust emission and fuel consumption figures to adjust the air-fuel ratio in spark-ignition internal combustion engines to values in the range of 1.05.ltoreq..lambda..ltoreq.1.4. Using the known Lambda sensors, sensor output voltages in the range (50 mV.gtoreq.U.sub.s .gtoreq.10 mV) result for this range of Lambda values. Because of the very low gradient of the sensor characteristic in this Lambda value range, even minor drifts of the control amplifiers which further process the sensor output signal result in major errors in the determination of the actual Lambda value. Assuming, for example, the actual Lambda value to be .lambda.=1.20, this corresponds to a sensor output voltage of approximately 20 mV. When comparing this sensor output voltage with the offset values of standard operational amplifiers as used in automotive electronics, such as: CA 3240 described in "RCA Linear Integrated Circuits" published 1978, SE 535 described in "Professionelle Integrierte Analogschaltungen 1981/1982" and LM 2902 and LM 224A described in "Linear Databook" of National Semiconductor Corporation, it will be seen that the total drift as the sum of the offset voltage and the offset voltage drift is between 2 mV and 10 mV in the temperature range of between -40.degree. C. and +85.degree. C., that is up to 50% of the signal utilized. It will be apparent that such an arrangement will produce only unsatisfactory results regarding exhaust emission and fuel consumption figures. The use of high precision measuring amplifiers such as chopper amplifiers, in addition to involving high cost, is impaired by insufficient rigidity in the rough environment associated with motor vehicles and incompatibility regarding the supply voltages (mostly bipolar).